FIG. 4 illustrates an example of a conventional time interval measuring instrument ordinarily used to measure the time interval between input signals. FIG. 5 is the timing diagram of each signal recorded by the conventional time interval measuring instrument of FIG. 4.
The conventional time interval measuring instrument also has the ability to measure the phase difference between input signals. The conventional time interval measuring instrument generally is comprised of a measuring part 21, a calculation part 28, and a display 27. The measuring part 21 is made up of input terminals 11 and 12, capacitors 13 and 14, wave formatters 17 and 18, D-type flip flops 22 and 23, a signal period measuring circuit 24, and a time interval measuring circuit 25. The signal period measuring circuit 24 in the measuring part 21 and the time interval measuring circuit 25 have structures similar to those described in the Japanese Patent Publication No. 63-3272 and Japanese Patent Laying-Open Publication Nos. 62-294993 and 63-191970.
Analog signals S11 and S12 are input to the input terminals 11 and 12, respectively and the DC components of the analog signals S11 and S12 are removed by the capacitors 13 and 14. The analog signals S11 and S12 are then input to the wave formatters 17 and 18, where the threshold voltage is set at 0 V. In the wave formatters 17 and 18, the analog signals S11 and S12 are changed to square waves S21 and S22. The signals S21 and S22, thus formatted to square waves, are input to the measuring part 21 through the input terminals 19 and 20. The signal period P of signal S21 as shown in FIG. 5 (which, in many instances, is the same as the signal period of signal S22) is measured by the signal period measuring circuit 24. Also, the time interval T between the rising edge of signal S21 and the rising edge of signal S22, as shown in FIG. 5, is measured by the time interval measuring circuit 25. The calculation part 28 receives the measured signal period P and time interval T from the measuring part 21 and instructs the display 27 to display the measured information of period P and time interval T. At the same time, the calculation part 28 performs a calculation of the phase difference between the two signals [(T/P).times.360 degree], which is also displayed on the display 27.
The above mentioned conventional instrument measures the time interval T between the signals S21 and S22 by assuming that the signal propagation time .tau.1 from the input terminal 11 to the input terminal 19 and the signal propagation time .tau.2 from the input terminal 12 to the input terminal 20 are equal. However, in practice, it is difficult to keep the difference between signal propagation times less than 100 pico-seconds because of the differences in accuracy between electric devices (such as the wave formatters 17 and 18). Furthermore, the signal propagation times may be affected by the frequency or amplitude of the input signals, or other conditions such as temperature. Therefore, a disadvantage of the conventional time interval measuring instrument is that highly precise time measurement is not possible, since an instrument requirement is that the propagation times of the two signal paths be equal. Even though the propagation time can be forced to stay equal under certain conditions, it is practically impossible to keep them equal under all conditions.